Sealing anodic films

ABSTRACT

Fabrication of corrosion resistant materials. Corrosionresistance of anodized aluminum is promoted by sealing the anodized surface film in nickel acetate, followed by sodium dichromate, and keeping the temperature of these solutions-mainly that of the acetate-substantially below the boiling point and particularly at 160* to 180* F. The product is characterized by firm bonding of chromate ions to aluminum in the anodic film.

United States Patent References Cited OTHER REFERENCES Modern Electroplating by Frederick Lowenheim, Second Ed. 1963, p. 638

Metal Finishing, August 1960, Pearlstein, pgs. 40- 41 Primary ExaminerJohn H. Mack Assistant Examiner-R. L. Andrews Attorney-Carl l-l. Synnestvedt ABSTRACT: Fabrication of corrosion resistant materials. Corrosion-resistance of anodized aluminum is promoted by sealing the anodized surface film in nickel acetate, followed by sodium dichromate, and keeping the temperature of these solutions-mainly that of the acetate-substantially below the boiling point and particularly at l60to I80 F. The product is characterized by firm bonding of chromate ions to aluminum in the anodic film.

' SEALING ANODIC FILMS BACKGROUND OF THE INVENTION In the sealing of anodic films on aluminum, excellent results have been obtained by first immersing the film in a hot solution of nickel acetate and thereafter immersing it in a hot solution of sodium dichromate. However, still further improvement of the sealing process, and of the corrosion-resistance obtained thereby, is needed for a variety of applications of aluminum members.

The invention herein described was made in the course of a contract with the Department of the Air Force.

NATURE OF THE INVENTION It has been discovered that substantial, further improvement can be obtained when the anodized material is immersed in the acetate, and preferably also in the dichromate, at temperatures kept distinctly below the boiling point, although raised substantially above room temperature. The improved process is advantageously applied to aluminum sheets exposed to serious dangers of corrosion. While the new method has broader applicability, it is particularly useful under conditions where the treated aluminum is subject to contact with salt, or to certain rocket fuel and oxidizer products, notably those including moist vapors of nitrogen peroxide, N either in the atmosphere or in space.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an enlarged, fragmentary, cross-sectional view of a piece of anodized aluminum to be subjected to the new sealing process.

FIG. 2 is a similar view showing the same piece after performance of the acetate sealing step in the new process.

FIG. 3 is a similar view showing the product on completion of the dichromate sealing step of this process.

FIG. 4 is a flow chart showing the conditions under which the acetate and dichromate are applied in accordance with the invention.

DETAILED DESCRIPTION In FIG. 1, numeral designates a surface portion of a sheet of aluminum alloy, for example an alloy of the type commercially known by number 2024T3 or 7075T6 or 7178-T6 or 7079-T6, which has been anodized by immersion in a bath of sulphuric acid and which as a result thereof has an anodized surface layer or anodic film 11. In accordance with the invention the film can be kept fairly thin, and can be produced by sulphuric acid treatment during substantially less than the 30 minutes formerly usual in certain cases; this use of a thin film not only facilitates the process but also produces improved fatigue properties of the aluminum alloy. In many cases the aluminum sheet has 0.04 inch thickness, and it is to be noted that the drawing, being enlarged, represents only an upper portion thereof.

In accordance with the invention the anodized aluminum or alloy is sealed, in order to close the holes and cracks which are present in the anodic film, and the sealing is initiated by immersing the product of FIG. I in a solution of nickel acetate which, in contrast to former practice, is kept at an elevated temperature, well above room temperature, but not in excess of about 180 F. Physical and statistical evaluation of factorial designed experiments with the new method and comparative methods shows that by far the most advantageous results are obtained by acetate-sealing at temperatures below 180 F. In view of this finding and general pertinent experience, new and improved corrosion protection has been obtained by using acetate-sealing temperatures between about 160 and 180, and preferably about 170 F. followed by the dichromate sealing step.

It has been found advantageous to prepare the acetate solution with 1 to 1.5 percent-preferably 1.25 percent-of nickel acetate, by weight, and to immerse the anodized material in it for 10 minutes, although immersion times of 5 to l5 minutes also were acceptable, and in certain case advantageous. The nickel acetate, Ni (C,H 0 was freshly prepared from reagent grade chemicals and was dissolved in water highly purified by deionization.

The operation leads to some swelling of the anodic film, as has been noted schematically by broken lines with respect to acetate-sealed film 12 (see FIG. 2), compared with unsealed film 11 (see FIG. 1).

The acetate sealing is followed by the second step, wherein sodium dichromate, Na Cr,0,, is applied. Excellent results have been obtained by applying it as a solution in the range of 3 to 7 percent by weight but best results are available in the preferred range of 3.8 to 4.5 percent, with deionized or equivalently purified water as a solvent. Again it was found possible or advantageous to keep the temperature of the solution relatively low, but in this case a somewhat wider range of temperatures, reaching from to 212 F. was found to be proper, pursuant to the acetate sealing as described. It was found sufficient to apply the chromate treatment for about 2 minutes, although immersion times within the range of onehalf to 10 minutes were found conducive to good results. The dichromate sealing causes further swelling of the acetatesealed film l2, producing ultimate film 13 (FIG. 3).

There is much demand for materials of great corrosion-resistance, not only in rocket technology but also in connection with various automotive products and commercial appliances. The product of the new process is very superior to the materials available thus far, wherever aluminum is needed which has a maximum of resistance to highly corrosive agents. At the same time, expense is kept low by the low-temperature seal solutions characterizing this invention, and the thin anodic films, low-concentration sealing solutions and short exposure times preferably used.

While the exact constitution of the material, causing its superior corrosion resistance, cannot be observed by any methods presently known, it has been determined by X-ray scintillation tests that chromate ions are bound to the film, and it is thought particularly to aluminum oxide in it, in unusually large numbers and by unusually strong bonding forces.

Iclaim:

l. A process of sealing a film, produced by anodizing aluminum with sulphuric acid, said process comprising: immersing said film in an aqueous solution of nickel acetate maintained at a temperature substantially in excess of room temperature and not substantially above 180 F.; and thereafter immersing said film in an aqueous solution of sodium dichromate maintained at a temperature substantially in excess of room temperature and not above 212 F.

2. A process as described in claim 1, and further characterized by maintenance of the temperature of said solution of nickel acetate in the range between about 160 F. and about 180 F.

3. A process as described in claim 1, and further characterized in that said aqueous solution of nickel acetate consists of from 1 to 1.5 percent of nickel acetate in highly purified water.

4. A process as described in claim I, and further characterized in that said aqueous solution of nickel acetate consists of approximately 1.25 percent of nickel acetate dissolved in deionized water kept at approximately F., and in which process the aluminum, with the film thereon, is immersed in said solution of nickel acetate for approximately 10 minutes.

5. A process as described in claim I, wherein the aqueous solution of sodium dichromate consists of from 3 to 7 percent of sodium dichromate dissolved in deionized water kept in the range from about 160 F. to about 212 F., and in which process the aluminum, and the acetate-exposed film thereon, is immersed in said solution of sodium dichromate for approximately one-half to 10 minutes.

6. A process as described in claim 2, and further characterized by maintenance of the temperature of said solution of sodium dichromate in the range between about 160 F. and about 2 12 F.

7. A process as described in claim 2, and further characterized in that the aluminum, with the film thereon, is immersed in said solution of nickel acetate for approximately minutes.

8. A process as described in claim 3, and further charac- 5 terized in that the aluminum, with said film thereon, is immersed in said solution of sodium dichromate for approximately 2 minutes.

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2. A process as described in claim 1, and further characterized by maintenance of the temperature of said solution of nickel acetate in the range between about 160* F. and about 180* F.
 3. A process as described in claim 1, and further characterized in that said solution of nickel acetate consists of from 1 to 1.5 percent of nickel acetate in highly purified water.
 4. A process as described in claim 1, and further characterized in that said aqueous solution of nickel acetate consists of approximately 1.25 percent of nickel acetate dissolved in deionized water kept at approximately 170* F., and in which process the aluminum, with the film thereon, is immersed in said solution of nickel acetate for approximately 10 minutes.
 5. A process as described in claim 1, wherein the aqueous solution of sodium dichromate consists of from 3 to 7 percent of sodium dichromate dissolved in deionized water kept in the range from about 160* F. to about 212* F., and in which process the aluminum, and the acetate-exposed film thereon, is immersed in said solution of sodium dichromate for approximately one-half to 10 minutes.
 6. A process as described in claim 2, and further characterized by maintenance of the temperature of said solution of sodium dichromate in the range between about 160* F. and about 212* F.
 7. A process as described in claim 2, and further characterized in that the aluminum, with the film thereon, is immersed in said solution of nickel acetate for approximately 10 minutes.
 8. A process as described in claim 3, and further characterized in that the aluminum, with said film thereon, is immersed in said solution of sodium dichromate for approximately 2 minutes. 